My limited experience, from a car, of the road from Tijuana down to Rosarito, and a day spent around Rosarito, is that the area is generally pretty flat and could easily be negotiated on any fixed gear bike. Strava seems to bear this out, with no categorized climbs inside the downtown area, and shortish, gradual climbs (</=2km, </=6%) to the uphill areas to the north. It definitely seems from other responses that Liege would have the greater need for lower gears.